This invention relates to active noise control (ANC) systems that are used for attenuating undesirable noise, and more particularly, to ANC systems utilized for canceling noise produced by internal combustion engines, where the noise contains multiple closely spaced sinusoidal frequency components having amplitudes and frequencies that vary in relationship with the rotational speed of the engine.
Conventional active noise control (ANC) systems attenuate undesirable noise by producing and superimposing noise canceling waves, which are substantially equal in amplitude and frequency content but shifted 180 degrees in phase with respect to the undesirable noise. As used in the present specification and the appended claims, the term noise is hereby defined to include both acoustic waves and mechanical vibrations propagating from a noise source.
Recently, ANC has been accomplished by employing modern digital signal processing and adaptive filtering techniques. Typically, an input sensor is utilized to derive a signal representative of the undesirable noise generated by a source. This signal is then applied to the input of an adaptive filter and is transformed by the filter characteristics into an output signal used for driving a cancellation transducer or actuator such as an acoustic speaker or electromechanical vibrator. The speaker or vibrator produces canceling waves or vibrations that are superimposed with the undesirable noise generated by the source. The observed or residual noise level resulting from the superposition of the undesirable noise and the canceling waves is then measured with an error sensor, which develops a corresponding error feedback signal. This feedback signal provides the basis for modifying the characteristics of the adaptive filter to minimize the overall level of the observed or residual noise.
Such systems have been successfully applied to attenuate, for example, repetitive noise generated by fans or electric motors and random noise propagating down heating and air conditioning ducts. The nature of acoustic and vibrational noise generated by an internal combustion engine differs quite significantly from the nature of the repetitive or random noise encountered in the past.
Engine generated noise generally contains a large number sinusoidal noise components having amplitudes and frequencies that are functionally related to the rotational speed of the engine. These frequency components have been found to be the even and odd harmonics of the fundamental frequency of engine half-order multiples or sub-harmonics interposed between the even and odd noise harmonics. Consequently, at low engine speeds, the difference in frequency between adjacent noise components (i.e. those noise components immediately preceding or following each other in the frequency domain) can become quite small, for example, as little as 5 Hz at engine idle. In addition, the amplitude, frequency, and phase of the engine generated noise components can vary quite rapidly in response to changes in engine rotational speed brought about by acceleration or deceleration of engine. Also, engines having differing numbers of cylinders generate noise characterized by different dominant frequency components due to the difference in their firing frequencies. Finally, engine generated noise can have different amplitude and frequency characteristics depending upon the particular type of noise, for example, acoustic noise propagating from the engine intake or exhaust system, or mechanical vibrations produced by operation of the engine, which are transmitted to a vehicle frame.
Consequently, there exists a need for a flexible active noise control system that can be tailored to effectively attenuate undesirable noise containing multiple sinusoidal frequency components, particularly in applications where the difference in frequency separating these noise components is small in comparison with the values of their individual frequencies, and where the amplitude, frequency, and phase of the sinusoidal noise components can change quite abruptly, such as in noise generated by an internal combustion engine during periods of rapid engine acceleration or deceleration.